maybe it’s the old “those who can’t do, teach.” Namely that all of the people at the top of their magic game have long left the Hogwarts ghetto behind.

not having read the books, though, I’ll leave it to you and John B. to duke it out.

We do see, in both film and book editions of the Potter series, the grown-ups reading, poring over old tomes, thinking deep thoughts. This is where they’re continually honing their skills. What might look like daydreaming, could well be mental exercise. It’s not visible because it is completely internal.”

That’s an extremely interesting and probably very valid point.
Though I might add the caveat that in Rowling the highest forms of magic are psychological as well, that is they might arise from the mind as a matter of technical competence, but draw their strength and force (dunamis) from the soul (psuchae).

However let me point out these instances of practice.

Teaching is practice. In order to teach properly you must demonstrate well and correctly and the teachers do this all of the time, if they are any good at what they do.

Real World – They practice by constantly engaging, the more important characters anyway, against their enemies and adversaries. That is a soldier trains for combat, or a cop for policing, but once trained most (most, not all) of their future training comes from exposure to real world situations which adds not only to their expertise but is superior training to merely static or simulated training. That is to say that one trains in order to master the real world, and real world experience is superior training to simulational training, because it is more fluid, dynamic, demanding, and difficult.

Athletics and Sports – The children are seen practicing all of the time by engaging in sporting activities and various other activates which require training, practice, and constant improvement.

Study – Many characters are seen studying many subjects in order to make discoveries which improve not only their abilities but their understanding and chances of success.

Contests – Contests abound for Harry and other students and several times similar historical contests (within the framework of the imaginary milieu of course) are mentioned between current and prior Wizards.

I suspect what you are really pointing out is lack of direct references to both “Workmanship Practice” – simulational training to maintain or improve skill sets, and “Enhancement Experimentation” the attempt to devise an experimental or training routine through research or hypothesis which might lead to a new discovery or an enhanced or augmented capability to one’s already established and mastered skill set(s). It’s not that practice is shown as lacking, but that practice as an attempt to specifically gain advantage or superiority or to develop novel capabilities is sparse.

In that case I have to agree that your observation in a basically good and astute one.
Such situations are basically lacking through most of the series and in one sense they would have vastly improved the survivability of the characters because it would have been an attempt to gain “advantage” and “technological advantage” (considering magic as a type of technology or sconce of expertise) over the enemies.

Only near the end of the series do the good guys really try to achieve a possible technological advantage to match the technology of subterfuge and magic which the enemy employs. You might say that on the part of the good-guys there is a notable lack of research and development with an attempt to gain technological or even skill-capabilities advantage. (It may be because of the fact that in that series the entire worldview is basically a back-wards looking framework, that skills and capabilities are “preserved from the past” rather than innovated with an eye towards the future. The present is tied directly to the past, there is even hostility towards change from many quarters in a wide spectrum of endeavours, instead of the future being the object of the present. Most people don’t think of Wizardry or Magic as an innovative, future leaning endeavor, but as an historical and psychological anachronism. That’s just a probability as to why the practice geared towards innovation and progress and advantage.)

But in any case I see your point.

The physical aspects–those that might be improved through practice–are quite simple. They can be learned to a degree of high competence through attendance at an educational institution such as Hogwarts (though there are those who pick them up without formal training).

The ‘umph’ in the magic is all intellectual. Literally, ‘It’s the thought that counts’.

We do see, in both film and book editions of the Potter series, the grown-ups reading, poring over old tomes, thinking deep thoughts. This is where they’re continually honing their skills. What might look like daydreaming, could well be mental exercise. It’s not visible because it is completely internal.

As you may have noticed, none of the adult wizards fluffs a spell, fumbles a wand. They’re maintaining their edge psychologically.

After all, does one need advanced technique in wielding a fork? Once you’ve got the technique down, you’ve got it. It’s even more basic than learning to ride a bike, a skill supposedly unforgettable.